Dry gas seal

ABSTRACT

A gas seal assembly comprises a pair of mutually rotatable sealing members A, B, each of which has a face adjacent a gap between the two members. One of the members B is urged in a direction attempting to close the gap h which constitutes a leakage path through the seal assembly. A gas bleed arrangement  7  allows gas from a high pressure side P 3  of the seal assembly to bleed into the gap at a position radially spaced between an inner diameter of the members and an outer diameter of the members A,B so as to apply a force tending to separate the members, the force decreasing as the size of the gap increases.

[0001] This invention relates to a gas seal between two parts betweenwhich relative rotation takes place (hereinafter referred to as“mutually rotating” or “mutually rotatable” parts).

[0002] A dry gas seal is a gas seal the operation of which does notdepend upon the supply of liquid lubricant, but which relies forlubrication upon the gas it is sealing. Dry gas seals have the advantageover liquid lubricated gas seals that there is no contamination of thegas with a lubricating liquid. This can be of particular importance inthe food and pharmaceutical industries. A dry gas seal offers a furtheradvantage over a liquid lubricated gas seal when it is used incombination with gas lubricated bearings. By lubricating both thebearings and seal with gas, rather than with lubricating oil, theconventional lubricating oil system of pump, filter and cooler thatwould otherwise be required can be dispensed with.

[0003] Dry gas seals are commonly used in gas compressors in the food,pharmaceutical and petroleum industries. Gases met with in the uses ofdry gas seals include air, natural gas, petroleum gas, carbon dioxideand other gases of high purity, such as gases used in anaesthesia.

[0004] The above comments regarding dry gas seals, and their commonapplications, are not intended to limit the scope of the claimedinvention. The comments are made by way of explanation only. Forexample, the hereinafter described and illustrated preferred embodimentsof dry gas seals could be used in other applications and/or inconjunction with gases other than those mentioned above.

[0005]FIG. 1 is a schematic sectional drawing of the general arrangementof a known dry gas seal assembly. The member A is attached rigidly tothe shaft 1. The member B is stationary but is free to slide axiallythrough a casing C. Leakage between a bore in the casing and the spigotof member B is prevented by an O-ring or similar seal D. High pressureof the gas within the casing is denoted by P₃ and the lower outerpressure by P₁. The purpose of the seal is to limit to an acceptably lowvalue the leakage from P₃ to P₁ whether the shaft is rotating orstationary. This is accomplished by arranging for the gap h whichseparates the active faces F₁ and F₂ of members A and B respectively tobe a few micrometers only.

[0006] The member B must be free to slide axially to accommodatedifferential expansion between shaft 1 and casing C. Because there is nopossibility of setting member B rigidly to form a gap h of a fewmicrometers, the gap has to be determined by the balance of the axialforces which act upon the axially free member B.

[0007] With reference to FIG. 1, the counterbalance forces which actupon member B from right to left are the force of the pressure P₃ actingupon the annular back face F3 of the member B, the force of pressure P₁acting upon the annular face F₄, the force of the compression springs atE and a small friction force of indeterminate sign arising from the sealD and also from the means (not shown) used to prevent member B fromrotating. The pressure forces do not vary with the axial position ofmember B and over the few millimetres of axial movement of member B theforce of the compression springs B is essentially constant. The outcomeis that the counterbalance force acting upon member B from right to leftis essentially independent of the axial position of member B and mustlie within a narrow band whose width is determined by the friction forceof indeterminate sign.

[0008] The necessary condition for the gap h to have a specified valuein the operation of a seal is illustrated by FIG. 2 in which theseparating force acting upon member B from left to right is illustratedby the sloping line 5. The counterbalance force acting on member B fromright to left is represented by line 2 or by line 3 in dependence uponthe sign of the friction forces whose double magnitude is represented bythe separation 4 of lines 2 and 3. These forces are plottedillustratively against the gap h. The specified value of gap h isdetermined by the intersection of the sloping line 5 with either line 2or line 3 in dependence upon the direction in which the friction forceis acting. It is closer to reality to consider the width 4 to be a bandof uncertainty so that all that is specified is that h lies somewherebetween the values of h given by the intersection of line 5 with lines 2and 3. For an intersection to exist, the separating force acting onmember B must depend upon the gap h and for gap h to be set stably theseparating force must fall as the gap h increases. The enabling matterin the production of a working dry gas seal is the arranging of aseparating force which decreases as the gap h increases. But for thatarrangement the faces of members A and B would be in dry contact andwould be damaged on rotation of the shaft 1 of FIG. 1.

[0009] At values of gap h of a few micrometers, the leakage flow of thegas between the active faces F₁ and F₂ Of FIG. 1 is viscous in nature.Ultimately as gap h is increased viscous flow changes to turbulent flow.Whilst viscous flow persists, and here for simplicity the facesF_(1 and F) ₂ of FIG. 1 will be taken as being plane parallel, then theway in which the pressure in the gap falls from P₃ at R₃ to P₁ at R₁ isindependent of the gap h. The continuous line 6 in FIG. 4 illustratesthe pressure distribution in the gap h of that condition. The separatingforce acting upon the member B from left to right is the area integralof the pressure distribution over the active face of member B from R₃ toR₁.

[0010] If the pressure distribution does not change with gap h, itfollows that the separating force does not change with gap h. Then therecan be no intersection of the separating and counterbalance forces ashas been shown to be essential and as is illustrated in FIG. 2.Consequently, the sealing surfaces of the FIG. 1 seal would not inpractice, provide a dry gas seal. There has to be an elaboration of theflow between plane faces to produce a separating force which falls asgap h increases.

[0011] According to a first aspect of the present invention there isprovided a gas seal assembly comprising a pair of mutually rotatablesealing members, each of which has a face adjacent a gap between the twomembers, one of the members being arranged in use to be urged in adirection attempting to close the gap which constitutes a leakage paththrough the seal assembly, further comprising a gas bleed arrangementfor allowing gas from a high pressure side of the seal to be admittedinto the gap at a position radially between inner and outer diameters ofthe members, the gas pressure in the gap applying a force in a directionto increase the separation of the two members, characterised in that thebleed arrangement comprises a channel having an associated flowresistance which is dimensioned such that the gas pressure at the gapend of the channel is reduced relative to the pressure at the highpressure side of the seal assembly and such that the gas pressure in thegap decreases as the size of the gap increases.

[0012] In dry gas seal assemblies of the type discussed generally above,it is possible that the sealing surfaces become, in use, other thanparallel to each other. It will be appreciated that such lack ofparallel is undesirable. It is desirable, that inclination between thefaces of the mutually rotating members should cause a moment which tendsto return the faces to their parallel condition. It is a further featureof the invention, therefore, to provide a dry gas seal assembly whereinupon departure from a parallel condition a moment is produced tending torestore that parallel condition.

[0013] According to a second aspect of the present invention there isprovided a dry gas seal assembly comprising a pair of mutually rotatablesealing members, each of which has a front face adjacent a gap betweenthe two members, the front face of at least one the member beingprovided with at least three pressure distributions formations eachseparately connectable via a bleed channel with a high pressure regionon one side of the seal assembly so that in the event that the face ofthe two members become non-parallel in use a force will be generatedtending to return the faces to their parallel condition.

[0014] According to a third aspect of the present invention there isprovided a dry gas seal assembly comprising a pair of mutually rotatablesealing members, each of which has a front face adjacent a gap betweenthe two members, one of the members being arranged in use to be urged ina direction tending to close the gap which constitutes a leakage paththrough the seal assembly, wherein one of the faces defining the gap isprovided with a circumferential undulation which is so constructed andarranged that, upon mutual rotation of the sealing members, a force isgenerated tending to separate the members, the magnitude of the forcebeing dependent upon the speed of mutual rotation of the sealingmembers.

[0015] As will be apparent from the following description and drawings,each of these different aspects of the present invention may be used inconjunction with any other aspect of the present invention, in a singleseal assembly.

[0016] The invention will now be described further, by way of example,with reference to the accompanying drawings, in which:

[0017]FIG. 1 is a schematic sectional drawing of the general arrangementof a known dry gas seal assembly;

[0018]FIG. 2 is a graph illustrating the requirement for the provisionof a suitable practically operating seal assembly;

[0019]FIG. 3 shows a first embodiment of a seal assembly according tothe present invention;

[0020]FIG. 4 is a graph showing the pressure in the gap between a pairof members of a seal assembly as a function of radius and showing howthe pressure varies as the width of the gap is increased;

[0021]FIG. 5 is a graph illustrating the relationship between theseparating force and the counterbalance force in the arrangement of FIG.3;

[0022]FIG. 6a shows the front face of a member of the first embodimentof FIG. 3 or the second embodiment of FIG. 7;

[0023]FIG. 6b illustrated a rear view of the member of FIG. 6a in thecontext of the second embodiment of FIG. 7;

[0024]FIG. 7 shows a second embodiment of seal assembly according to thepresent invention;

[0025]FIG. 8a shows a third embodiment of seal assembly according to thepresent invention;

[0026]FIG. 8b shows part of the front face of a member of the embodimentof FIG. 8a;

[0027]FIGS. 9a, FIG. 9b and FIGS. 9c represent a series of graphs,similar to FIG. 5, showing how the relationship between the separatingforce and the counterbalance force changes as the operating conditionsof the seal assembly vary;

[0028]FIG. 10a shows the front face of a member generally similar tothat in FIG. 6a, but additionally provided on its front surface with acircumferential undulation having six crests;

[0029]FIG. 10b is a side elevation of the member of FIG. 10a, showingthe circumferential undulation in the front surface of the member;

[0030]FIG. 11 shows the front face of a modified member of the sealassembly of FIG. 8, the left and right hand halves of FIG. 11 showingdifferent variants of modification;

[0031]FIG. 12a is a schematic top hand view of a means for producingundulation in the front face of a member, such as the member of FIG. 10aand FIG. 10b; and

[0032]FIG. 12b is a sectional side elevation along the line of XII-XIIin FIG. 12a.

[0033] It is convenient here to describe in general terms the means ofcreating a separating force which falls as gap h increases.

[0034]FIG. 3, which illustrates a first embodiment of a seal assembly ofthe invention, is similar to FIG. 1, but with the addition, forillustrative purposes, of three or more arcuate, discontinuousdistribution grooves 3, at radius R₂, in the active face F₁ of themember A and with the addition for illustrative purposes of a bleedchannel 7 and an inline bleed flow throttle or resistance G associatedwith each arcuate groove J and which puts each groove J intocommunication with the high pressure P₃ within the casing. In terms ofphysical principles, the arcuate grooves 3, channels 7 and the bleedresistances G might equally well be features of the stationary member B.

[0035] The significance of the distribution grooves J is to denote thatthe pressure on the concentric circle of radius R₂ is constant, but onlywhen the faces F₁ and F₂ are parallel. The pressures must become unequalwhen the faces are not parallel in order to produce the moment.

[0036] Two extreme conditions will now be considered with reference toFIG. 4. In the first extreme, the gap h in FIG. 3 is so small that theflow resistance through the gap h is large in comparison with the flowresistance through the bleed resistance G. Then the pressures in the gaph at R₃ and at R₂ are both essentially equal to P₃ and in essence all ofthe pressure drop from P₃ to P₁ within the gap h occurs within theradial interval R₂ to R₁. This pressure distribution in gap h is shownillustratively by the dashed line 8 in FIG. 4. In the second extreme,the gap h has increased to a point where the flow resistance through thegap h is small in comparison with the flow resistance of the bleedresistance G. Then as gap h increases the flow through the bleedresistance G becomes progressively less relevant and the pressuredistribution approaches asymptotically the distribution of thecontinuous line 6 in FIG. 4. As described previously that continuousline 6 pertains to unelaborated plane faces and is independent of h. Bythe provision of the groove J, the bleed channel 7 and bleed flowresistance G a pressure distribution is thus created which varies withthe gap h. It follows that those provisions also create a separatingforce which falls as the gap h increases and thereby satisfies theenabling requirement illustrated by FIG. 2. FIG. 5 illustrates theeffect of the bleed upon separating force as a function of the gap h.

[0037] It is this monotonic reduction in separating force as the gap hincreases (and the concomitant increase in separating force as the gapreduces) that is elsewhere in this specification referred to as theseparating force varying “inversely” with the size of the gap. The term“inverse” is not being used in the context of a precise mathematicalrelationship of the force varying in strict proportion to the reciprocalof the gap.

[0038] The bleed of gas to groove J, via the channel 7 and flowresistance G, is a flow additional to that which would otherwise leakbetween the plane F₁, F₂.

[0039] It cannot, in practice, be assumed that the active faces F₁ andF₂ of the FIG. 1 arrangement will be parallel and it is desirable thatany inclination of one to the other should invoke a restoring moment.

[0040] A restoring moment can be created by establishing in the gap hthree or more arcuate sectors each of which responds to some weightedvalue of gap h over its sector. FIG. 6a is a plan view of the activeface F₁ of the member A of FIG. 3, showing three distribution grooves Jformed therein, rather than a single continuous 360° groove. Each grooveJ is in communication with the high pressure P₃ via a channel 7 and thenvia equal bleed resistances G. The three distribution grooves J arecapable of sustaining unequal pressures and divide the face F₁ intothree sectors. The pressures in the grooves J respond in an inversefashion to the weighted average value of gap h over the sectors andcause the axial separating force or each sector also to respond in aninverse fashion. In that way an inclination of the faces F₁ and F₂ ofmembers A and B one to another is caused to invoke a restoring moment.

[0041] What is described in conjunction with FIG. 6a is the formation inthe active face of either the rotating member A of FIG. 3 or in thestationary member B of FIG. 3 of a number, equal to or exceeding three,of equal and equally disposed distribution grooves J with eachdistribution groove J put into communication with the high pressure P₃within the casing via an equal bleed resistance of appropriate value andof any practicable physical form.

[0042]FIG. 7 is a cross-section of a second embodiment of a sealassembly. The rotating member A is elaborated in FIG. 7 with the annularmembers A₁ and A₂. The faces of both members are parallel. Both faces ofmember A₂ are lapped flat, as also is the right hand face of member A₁.The facing faces of members A₁ and A₂ are held in contact by a screwedring L. Leakage past the right hand surface of the flange A and the lefthand surface of the member A₁ is stopped by a seal M. The member A₂ hasformed in it distribution grooves as indicated at J and communicatingholes as indicated at K, as described with reference to FIG. 6a. A bleedresistance or throttle referenced G in FIG. 7, comprises a generallyradial depression 9 of depth h₁ etched or otherwise produced in eitherthe right hand face of member A₁ or in the left hand face of member A₂.The plan of such a depression 9 is illustrated in FIG. 6b. One suchdepression 9 is provided in alignment with each of the communicatingholes K.

[0043] Attributes of this second embodiment include an insensitivity touniform wear of the active faces F₁ and F₂ and that for a desired valueof bleed resistance in total the small number of depressions 9 which aredemanded allows a greater value for the depression depth h₁ than agreater number of depressions would permit. In consequence thedepressions of small number are less prone to blockage by particulatematter. Furthermore in comparison with another embodiment which followsbelow (and with other things equal), the bleed resistances areunaffected by wear of the active faces F₁ and F₂ at which relativerotation occurs and the bleed value of the resistances is constant andindependent of the gap h between the active faces. With other thingsequal, the effect or this is to cause the separating force to fall moresharply as the gap h increases.

[0044]FIG. 8a, which is generally a repetition of FIG. 7, illustrates athird embodiment of a seal assembly of the invention in which the bleedresistances G are formed in the right hand face of member A₂ and takethe form of a single depression per distribution groove J. In thisembodiment the bleed resistances decrease as gap h increases becausetheir effective depth is depression depth h₁ plus gap h. Detritus fromwear of the active faces might accumulate in the depressions of thebleed resistances and alter their value and furthermore wear of theright hand face of member A₂ will reduce the value of h₁ and if the wearshould progress sufficiently the bleed resistances would be removed. Apositive attribute of this embodiment is the absence of the member A₁ ofFIG. 7 and a simpler manufacture.

[0045] The principle which has been described of bleed resistancescommunicating with the high pressure side to distribution grooves in oneor other of the active faces of a seal applies equally if the pressureP₁ of FIG. 1 is the high pressure and the pressure P₃ of FIG. 1 is thelow pressure. Changes in detailed design are then required but theprinciple of operation of the seal remains.

[0046] The invention is not limited to the precise details of theforegoing. For example, the actual separation force across the seal canbe used independently of the invention described in relation to FIG. 6in which a lack of axial alignment causes a restoring force. Of course,these two features can be used together if desired.

[0047] The formations can be provided in the stationary or thenon-rotating part of the seal assembly. Of course, the seal assembly canbe equally applicable to a pair of members which are both rotating,although one is rotating faster or in the opposite direction to theother so that there is still mutual rotation between the two parts.

[0048] In the above described embodiments of a seal assembly of theinvention, the determination of the gap at the design point of the sealis illustrated in FIG. 5. Within the circumstances in which dry gasseals operate, where a bleed is provided as in the above described andillustrated seal assemblies, the separating force reduces as the gap hbetween the active faces increases.

[0049]FIGS. 2, 4 and 5 above pertain to the operating point of the abovedescribed and illustrated seal assemblies when the pressure P₃ hasreached its design operating value.

[0050] The series of FIGS. 9a, 9 b and 9 c, illustrate a range ofconditions. FIG. 9a pertains to the operating point of a seal where thepressure P₃ has reached its design operating value. FIG. 9b representsthe condition where the pressure upstream of the seal is only 75% of thedesign pressure P₃. FIG. 9c represents the condition where the upstreampressure is only 10% of the design pressure P₃.

[0051] In the FIG. 9b condition there will still be a gap between theactive surfaces of the seal assembly, but this gap is at a value G₂which is less than the gap value G₁ at the counterbalanceforce/separating force intersection in FIG. 9a. Nevertheless, in theFIG. 9b condition there will still be a gap, albeit reduced, between theactive surfaces.

[0052] In the FIG. 9c condition, however, it can be seen that there isno intersection between the counterbalance force line and the separatingforce line. Consequently, in this condition there will be no gap betweenthe described and illustrated seal assemblies with the result that theactive surfaces will be in contact. The graphs of FIGS. 9a-9 c show whatcan happen with the embodiments of FIGS. 3, 7 and B when running belowdesign values.

[0053] In FIGS. 9a, 9 b and 9 c, the force scales have false origins andhave different scaling factors. Nevertheless, the general trend isapparent that as pressure P₃ falls the intersection of thecounterbalance force with the curve of the separating force movesprogressively so as to provide a smaller gap between the active surfacesuntil there is eventually no gap.

[0054] The hereinafter described and illustrated embodiments of a sealassembly of the invention are concerned with giving protection to theseal assembly when operating with a pressure P₃ below the designoperating value. The gap between the active faces of a prior art seal isa function of the pressures P₁ and P₃ and is for all practical purposesindependent of rotational speed.

[0055]FIGS. 9c to 9 a (in that order) may be regarded as generallyillustrative of the condition of a gas compressor seal as the compressoris started up and its speed increased up towards it design operatingspeed. From initial start-up to somewhat above one third of full speed,there will be no gap between the opposing active surfaces, whichsurfaces will be in contact.

[0056] Typically, the rotating member of a gas seal has a very hardceramic surface, whilst the stationary member is made of graphite. Withsuch a combination, the seal is able to resist transient contact of thesurfaces at low speed without significant damage, whilst the compressoris being brought up to speed. Because, however, a gap is a matter ofpressure and not of speed, the transient contact will not occurfrequently even though the compressor may be stopped or slowedfrequently in operation. Provided that the casing of the compressor andseal remain subject to a high pressure maintained in the receiver of thecompressor, there should be no contact.

[0057] The hereinafter described and illustrated arrangements mitigatethe potential seal active surface contact problems noted above byproviding a speed dependent increment in the active surface separatingforce. By way of illustration, if one takes the situation shown in FIG.9c, providing a sufficient speed dependent increment in the separatingforce would raise the separating force curve from the position shown inFIG. 9c so that this curve intersects with the counterbalance force toprovide a finite gap between the active seal surfaces. More generally,adding a significant speed dependent increment to the separating forcewill provide a finite gap at a lower speed of rotation than wouldotherwise be the case. This speed dependent increment will besignificant when the pressure upstream of the seal is small, as in thesituation represented by FIG. 9c, but it will become progressively lesssignificant as the upstream pressure rises, to become insignificant whenthe upstream pressure is the high design operating pressure representedin FIG. 9a.

[0058] To provide a significant speed dependent increment in theseparating force, one of the active surfaces of the seal assembly isprovided with a circumferential undulation.

[0059] Before going on to disclose how this might be applied to a sealassembly of the invention, a brief background explanation might beuseful. When one has two annular plates with flat faces, for example aswith the contact plates in a vehicle clutch, the plates have a commonaxis and one plate is rotated whilst the other is stationary. When thefaces mate, the lubricant between the plates is expressed and the platesslide against each other in dry contact. If, however, one were toprovide the surface of one of the plates with a circumferentialundulation, then these surfaces would not mate. Some lubricant would beretained between the surfaces. Furthermore, the relative motion betweenthe plates would generate pressures tending to separate the plates aslubricant is dragged over the crests of the undulations. This wouldapply even when the lubricant is gas. This general principle is employedin the following arrangements.

[0060] In the context of the embodiments of a seal assembly of theinvention discussed above, providing a precautionary measure forameliorating contact could, in one arrangement, involve taking themember illustrated in FIG. 6a and providing on its front surface F₁ (thesame active surface provided with distribution groove J) acircumferential undulation, for example having an amplitude of 3micrometers, providing an undulation depth (referenced S in FIG. 10b)from crest to trough of 6 micrometers. In the FIGS. 3 and 7 embodiments,for example, modifying the member of FIG. 6a in the above discussedmanner would result in the right hand (or front) face F₁ being providedwith the circumferential undulation. FIG. 10a is a plan view, generallysimilar to that of FIG. 6a, additionally showing the front (active)surface F₁ of the member as being provided with a circumferentialundulation. The cross hatched areas of FIG. 10a denote the crests orpeaks of the undulations. In FIG. 10b, which is a side elevation of themember of FIG. 10a, the undulations are exaggerated for reasons ofclarity. Upon rotation of the members on which the surface F₁ isprovided in the FIG. 3 or 7 embodiment, the provision of undulations inthe face F₁ will produce a self-generated increment to the separatingforce. The undulations will not be entirely in addition to the gap underthe conditions that a uniform finite gap would otherwise exist, but willbe partly subsumed within the gap. Consequently, the increase in leakagearising from the provision of the undulations becomes lessproportionally as the pressure across the seal increases.

[0061] Instead of being provided with smooth undulations as in FIG. 10,the face of a plate may instead be provided with undulations in the formof discrete sharp shallow depressions of small arcuate width, as will beexplained later.

[0062] To modify the FIG. 8 arrangement to provide it with aprecautionary measure for ameliorating the contact problem discussedabove, the preferred means would be to replace the single depressionsper arcuate distribution groove J forming the bleed resistances G in theright hand (front) face of the member A₂ with a multiplicity ofchannels, for example as shown by the reference numeral 11 in the righthand half of FIG. 11. Although the single depressions forming the bleedresistances G in the unmodified FIG. 8 arrangement will themselvesproduce a self generated increment to the separating force, that effectcan be enhanced by the provision of a plurality of channels 11 perdistribution groove J, which channels will be less deep if the seal isto retain the same leakage rate. These channels 11 might readily beproduced by etching. Although the channels 11 illustrated in the righthand half of FIG. 11 as running between the distribution groove J andthe circumferential extremity of the member, which replace the deeperdepression forming the bleed resistance G in the FIG. 8 arrangement,occupy a greater arc than the deeper radially extending groove in theFigure B arrangement, the total flow resistance is kept substantiallythe same because of the reduced depth of the channels 11 in the righthand half of FIG. 11 relative to the single depression G perdistribution groove J illustrated in FIG. 8b.

[0063] By way of explanation, if the plate illustrated in the right handhalf of FIG. 11 were to be in contact with the opposing acting surfaceof the seal assembly, such that the channels 11 in the right hand halfof FIG. 11 became the only means of gas flowing through the periphery ofthe member into the arcuate distribution groove J, then with otherthings being equal, the volume flow through a channel is proportional tothe cube of its depth, multiplied by its angular width. The significanceof this is that a relatively small decrease in depth of a depressionallows for a considerably greater angular width to be employed for thereduced depth depression whilst leaving the flow therethroughunaffected. Thus, if the single depression running to the distributiongroove J illustrated in FIG. 8b were to be 10 micrometers in depth, andthe depth of the replacement, reduced depth channels 11 illustrated inthe right hand half of FIG. 11 were to be of 6 microns in depth, thenfor the same flow, the reduced depth channels 11 can occupy in total anarc of approximately 5 times the arc occupied by the single, deeperradial depression forming the bleed resistances G shown in FIG. 8b.

[0064] The left hand half of FIG. 11 illustrates an alternative to themodification illustrated in the right hand half of FIG. 11, whichalternative modification would also be appropriate for the FIG. 8embodiment of seal assembly. In the left hand FIG. 11 arrangement, sevenof the channels 12 do not extend radially inwardly sufficiently as tolink with the distribution groove J. Consequently, for these sevenchannels 12 there is a barrier that will diminish the flow of gas fromthe circumferential periphery of the seal to its distribution groove Jas a result of these seven depressions 12 being barred.

[0065] The kind of undulations illustrated at 11 and at 12 in FIG. 11themselves provide the necessary bleeds from the high pressure side of aseal to its distribution grooves and therefore their useful applicationis limited to seals generally of the type illustrated by FIG. 8. Ifapplied to seals of the type illustrated by FIG. 7 then the bleeds viachannels G and K would become unwanted and such applications aretherefore disadvantageous. However if the advantages listed previouslyof seals of the type illustrated by FIG. 7 are to be retained then ifthe depth from crest to trough of the undulations were no greater thanwould be the gap h in their absence at the design point of the seal,then the undulations become subsumed in the gap h. They add nothing tothe bleed of gas from the high-pressure side of a seal to itsdistribution grooves i.e. that essential bleeds remains the bleeds viachannels G and H.

[0066] A further property of the multiplicity of depressions illustratedon the right hand side of FIG. 11 is that they themselves in the absenceof distribution grooves J produce pressures resulting in a momentresisting the inclination of the active faces of a seal relative to oneanother. The arcuate distribution grooves could then be omitted withoutdisadvantage.

[0067] As mentioned above, depressions may be formed in the surface of aseal member by etching.

[0068]FIGS. 12a and 12 b illustrate one way in which to provide the faceof a seal member with a circumferential undulation, especially the sealmember illustrated in FIG. 10. FIG. 12a is a schematic top plan view ofa means for producing undulations in a seal member. FIG. 12b is asectional side elevation along the line XII-XII in FIG. 12a. In FIGS.12a and 12 b, a seal member blank 20 is shown, which seal member blankmight for example be used to produce the seal member illustrated in FIG.10a. Reference numerals 21 a, 21 b denote, respectively, inner and outerportions of a vacuum chamber 21 which can be evacuated, or partiallyevacuated, through a pipe 22. Six upstands are provided within thechamber 21 to support the reverse or back face of the seal member, i.e.not the face which will be an active face in use. These upstands 22 areshown dotted in FIG. 12a because they are obscured beneath the blank 20.The upstands 22 are lapped so that their upper surfaces lie accuratelyin a plane. As can most clearly be seen from FIG. 12b, the blank 20 isradially interposed in an annular space between the inner and outerportions 21 a and 2 bb of the chamber 21 with the blank 20 positioned onthe six upstands 22. Seal members in the form of O-rings 23 are providedto seal in between the inner and outer circumferential surfaces of theblank 20 and the inner and outer portions of the chamber 21 a, 21 b.

[0069] By evacuating or partially evacuating the chamber beneath theblank 20 the blank resting upon the upstands 22 will become distortedelastically in the six spans between adjacent upstands by the pressuredifference generated across the blank. The upper face of the blank(which is to form the active face in use) now undulates. By lapping thisupper face flat whilst the blank 20 is distorted and then releasing thevacuum or partial vacuum, the formerly flat upper face of the blank 20will be left with undulations complementary to those produced initiallyby the pressure difference. By means well known in the art, the pressuredifference may be calculated so as to produce undulations of a desiredamplitude. The active surface will vary continuously, but is unlikely tohave a surface varying strictly harmonically from its mean plane due tothe method of production of the undulations.

[0070] If a pressure difference greater than one atmosphere is requiredto produce the desired amplitude of undulation, then the assembly ofvacuum chamber 21 and blank 20 could be placed in a pressure vesseltogether with the means for lapping the upper (active) surface of theblank so that a pressure difference greater than one atmosphere can beestablished across the blank.

1-25 (cancelled).
 26. A gas seal assembly comprising: a pair of mutuallyrotatable sealing members, each of which has a front face adjacent to agap between the members, one of the members being arranged in use to beurged in a direction attempting to close the gap which constitutes aleakage path through a seal assembly; and a gas bleed arrangement forallowing gas from a high pressure side of a seal to be admitted into thegap at a position radially between inner and outer diameters of themembers, a gas pressure in the gap applying a force in a direction toincrease the separation of the two members, characterized in that thegas bleed arrangement comprises a channel having an associated flowresistance which is dimensioned such that the gas pressure at a gap endof the channel is reduced relative to the pressure at the high pressureside of the seal assembly and such that the gas pressure in the gapdecreases as size of the gap increases.
 27. The gas seal assembly ofclaim 26, wherein the gap is a few micrometers.
 28. The gas sealassembly of claim 26, wherein a first one of the members is associatedwith a rotatable shaft and a second one of the members is associatedwith a casing.
 29. The gas seal assembly of claim 28, wherein the secondone of the members is biased by a spring in a direction tending to closethe gap.
 30. The gas seal assembly of claim 29, further comprising aseal ring for sealing between the second one of the members and thecasing.
 31. The gas seal assembly of claim 26, wherein one of themembers is in the form of a generally annular flange and the gas bleedarrangement comprises one or more channels extending from a rear face ofthe flange through to a front face of the flange adjacent to the gap.32. The gas seal assembly of claim 26, wherein the gas bleed arrangementfurther comprises one or more channels extending generally radially fromthe high pressure side of the seal assembly and formed in the front facein one of the members.
 33. The gas seal assembly of claim 32, whereinthe channels co-operate with formations in the front face to distributea bleed gas over the front face.
 34. The gas seal assembly of claim 33,wherein the formations comprise one or more generally arcuate orcircumferential grooves provided in the front face.
 35. The gas sealassembly of claim 26, wherein at least one front face defining the gapis provided with at least three pressure distribution formations eachseparately connectable via a respective bleed channel with a highpressure region on one side of the seal assembly so that in the eventthat each face of the two members becomes non-parallel in use a forcewill be generated tending to return each face to a parallel condition.36. The gas seal assembly of claim 35, wherein each of the threepressure distribution formations comprises a generally arcuate grooveprovided in the front face.
 37. The gas seal assembly of claim 36,wherein each of the bleed channels comprises a channel ending generallyradially from the high pressure side of the seal assembly, the channelbeing formed in the front face.
 38. The gas seal assembly of claim 36,wherein each of the bleed channels comprises a channel extending from arear face of one of the sealing members to the front face.
 39. The gasseal assembly of claim 36, wherein at least one of the sealing memberscomprises a front annular member and a rear annular member, a frontsurface facing the gap being formed on the front member and a rearsurface of the front member facing a front surface of the rear member,each bleed channel being formed in one or both of the front surface ofthe rear member and the rear surface of the front member.
 40. The gasseal assembly of claim 36, wherein each pressure distribution formationconstitutes a generally radially directed groove provided in the frontface, each bleed channel being formed by a radially outer portion of thegroove of its respective formation.
 41. The gas seal assembly of claim36, wherein the pressure distribution formations are spaced apartgenerally and symmetrically about the circumference of at least onemember.
 42. The gas seal assembly of claim 26, wherein one front faceadjacent to the gap is provided with a circumferential undulation whichis constructed and arranged, upon mutual rotation of the sealingmembers, to generate a force tending to separate the members, themagnitude of which force is dependent upon the speed of mutual rotationof the sealing members.
 43. The gas seal assembly of claim 42, wherein aplurality of generally radially extending grooves are formed in thefront face by the circumferential undulation.
 44. The gas seal assemblyof claim 42, wherein the circumferential undulation varies continuouslyaround a circumference of the front face.
 45. The gas seal assembly ofclaim 42, wherein the circumferential undulation comprises a pluralityof discrete generally radially extending grooves.
 46. The gas sealassembly of claim 43, wherein the grooves supply formations in the frontface, which formations distribute the gas flow over the front face. 47.The gas seal assembly of claim 43, wherein the formations comprise oneor more generally arcuate and/or circumferential grooves provided in asurface defining the gap.
 48. The gas seal assembly of claim 46, whereinonly some of the generally radially extending grooves extend into thegenerally arcuate or circumferential grooves.
 49. The gas seal assemblyof claim 46, further comprising one or more passageways extending from arear face of the member in whose front face the formations are providedto the front face for the supply of high pressure gas through thechannels to the formations.